In a conventional thin-film solar battery (thin-film photoelectric converter), thin-film solar battery cells (thin-film photoelectric conversion cells) are configured, for example, by forming a thin-film semiconductor layer serving as a photoelectric conversion layer on a substrate having a light-transmitting electrode formed thereon and forming a reflecting conductive film on a rear side. These solar battery cells generate photo-electromotive force when light is incident from the front side (light-transmitting electrode side).
The thin-film solar battery is configured such that adjacent ones of a plurality of thin-film solar battery cells are spaced apart by a predetermined distance, and the thin-film solar battery cells are electrically connected in series. The photoelectric conversion layers between adjacent thin-film solar battery cells are electrically isolated from each other.
Such a thin-film solar battery is manufactured by the following method. First, an insulating light-transmitting substrate which has a transparent electrode layer made of a transparent conductive oxide (TCO) such as tin oxide (SnO2) or zinc oxide (ZnO) formed thereon is prepared, and the transparent electrode layer has an irregular texture structure formed on its surface. The transparent electrode layer is cut and removed by irradiation with a laser beam so as to be processed into a stripe shape. The texture structure has a function of scattering sunlight incident on the solar battery to improve the efficiency of light utilization in the thin-film semiconductor layer. Next, a photoelectric conversion thin-film semiconductor layer made, for example, of amorphous silicon is formed on the transparent electrode layer by, for example, plasma CVD (Chemical Vapor Deposition).
Subsequently the thin-film semiconductor layer is cut and removed by applying a laser beam to positions different from the positions at which the transparent electrode layer has been cut so as to be processed into a stripe shape. Next, a rear-side electrode layer made of a light-reflecting metal is formed on the photoelectric conversion thin-film semiconductor layer by, for example, sputtering, and then the rear-side electrode layer is cut and removed by applying a laser beam to the positions different from the positions at which the transparent electrode layer has been cut so as to be processed into a stripe shape. In such a thin-film solar battery module, the photoelectric conversion cells processed into a stripe shape are connected in series to generate a high output voltage.
In such a thin-film solar battery, the quality of the thin-film semiconductor layer serving as the photoelectric conversion layer largely controls the power generation efficiency of the battery. For example, when the thin-film semiconductor layer is a silicon film, the density of structural defects present in the silicon film must be about 1×1015/cm3 to achieve sufficient power generation efficiency. Impurities present in the silicon film also reduce the power generation efficiency. In particular, when the thin-film semiconductor layer is a microcrystalline silicon film, the oxidation of the microcrystalline silicon film due to the ingress of oxygen into the silicon film significantly reduces the power generation efficiency.
In one method proposed to solve these problems, a microcrystalline silicon film in which the crystalline orientation in a direction perpendicular to the surface of a substrate is controlled is formed by, for example, plasma CDV to suppress oxidation of the silicon film (see, for example, Patent Document 1). More specifically, it has been found that the ingress of impurities is suppressed when the crystalline orientation is controlled such that the crystals are mainly oriented in (220). However, when the crystals are mainly oriented in (111), the ingress of impurities is facilitated, and the power generation efficiency is thereby reduced. Moreover, by increasing the degree of vacuum in a deposition chamber when the microcrystalline silicon film is deposited by plasma CVD, the ingress of impurities into the microcrystalline silicon film during deposition is suppressed.